System and method to automatically maintain electrical stimulation intensity

ABSTRACT

Tissue stimulation systems generally include a pulse generating device for generating electrical stimulation pulses, at least one implanted electrode for delivering the electrical stimulation pulses generated by the pulse generating device, and a programmer capable of communicating with the pulse generating device. Stimulation pulses may be defined by several parameters, such as pulse width and amplitude. In methods of stimulating the tissue with the stimulation system, a user may adjust one of the parameters such as pulse width. The programmer may automatically adjust the pulse amplitude in response to the change in pulse width in order to maintain a substantially constant effect of the stimulation pulses.

CLAIM OF PRIORITY

This application is a continuation of U.S. application Ser. No.15/152,336, filed May 11, 2016, which is a continuation of U.S.application Ser. No. 11/553,447, filed Oct. 26, 2006, which is herebyincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to tissue stimulation systems and moreparticularly to a clinically-adaptive stimulation programmer.

FIELD OF THE INVENTION

One example of a stimulation system is a spinal cord stimulation (“SCS”)system. Spinal cord stimulation is a well-accepted clinical method forreducing pain in certain populations of patients. An SCS systemtypically includes an Implantable Pulse Generator (IPG) or aradio-frequency (RF) transmitter and receiver, electrodes, electrodeleads, and when necessary, lead extensions. The electrodes are implantedalong the dura of the spinal cord, and the IPG or RF transmittergenerates electrical pulses that are delivered, through the electrodes,to the dorsal column and dorsal root fibers within the spinal cord.Individual electrode contacts (the “electrodes”) are arranged in adesired pattern and spacing in order to create an electrode array.Individual wires within one or more electrode leads connect with eachelectrode in the array. The electrode leads exit the spinal column andattach to one or more electrode lead extensions, when necessary. Theelectrode leads or extensions are typically tunneled along the torso ofthe patient to a subcutaneous pocket where the IPG or RF-receiver isimplanted.

Spinal cord stimulators and other stimulation systems are known in theart. For example, an implantable electronic stimulator is disclosed inU.S. Pat. No. 3,646,940 issued Mar. 7, 1972 for “Implantable ElectronicStimulator Electrode and Method” that provides timed sequencedelectrical impulses to a plurality of electrodes. As another example,U.S. Pat. No. 3,724,467 issued Apr. 3, 1973 for “Electrode Implant forthe Neuro-Stimulation of the Spinal Cord,” teaches an electrode implantfor the neuro-stimulation of the spinal cord. A relatively thin andflexible strip of physiologically inert plastic is provided as a carrieron which a plurality of electrodes are formed. The electrodes areconnected by leads to an RF receiver, which is also implanted.

In U.S. Pat. No. 3,822,708, issued Jul. 9, 1974 for “Electrical SpinalCord Stimulating Device and Method for Management of Pain,” another typeof electrical spinal cord stimulation device is taught. The devicedisclosed in the '708 patent has five aligned electrodes, which arepositioned longitudinally on the spinal cord. Electrical pulses appliedto the electrodes block perceived intractable pain, while allowingpassage of other sensations. A patient operated switch allows thepatient to adjust the stimulation parameters.

An SCS system treats chronic pain by providing electrical stimulationpulses through the electrodes of an electrode array located at thedistal end of a lead placed epidurally next to a patient's spinal cord.The combination of electrodes used to deliver stimulation pulses to thetargeted tissue constitutes an electrode configuration. In other words,an electrode configuration represents the polarity, being positive,negative, or zero, and for certain SCS systems with such capabilities,relative percentage of the current or voltage provided through each ofthe electrodes. Electrode arrays used with known SCS systems may employbetween 1 and 16 electrodes on a lead. Electrodes are selectivelyprogrammed to act as anodes, cathodes, or left off, creating anelectrode configuration. The number of electrodes available, combinedwith the ability to generate a variety of complex stimulation pulses,presents a huge selection of electrode configurations and stimulationparameters (together referred to herein as “stimulation sets”) to theuser.

Other parameters that may be controlled or varied in SCS include thefrequency of pulses provided through the electrode array, pulse width,and the amplitude of pulses delivered. Amplitude may be measured inmilliamps, volts, etc., as appropriate, depending on whether the systemprovides stimulation from current sources or voltage sources. With someSCS systems, the distribution of the current/voltage across theelectrodes (including the case of the pulse generator or receiver, whichmay act as an electrode) may be varied such that the current is suppliedvia numerous different electrode configurations. In differentconfigurations, different combinations of electrodes may provide current(or voltage) in different relative percentages of positive and negativecurrent (or voltage). Moreover, there may be some electrodes that remaininactive for certain electrode configurations, meaning that no currentis applied through the inactive electrode.

Programming processes are described in U.S. Pat. No. 6,622,048, hereinincorporated by reference in its entirety. A stimulation programmer isutilized to instruct the pulse generating device to generate electricalstimulation pulses in accordance with selected parameters or stimulationsets. One known programmer for an IPG for SCS is called theBionicNavigator™, available from Advanced Bionics Corp., Sylmar, Calif.A stimulation programmer may be programmed by a technician attending thepatient.

A stimulation programmer may be used in several scenarios. For example,when an SCS system is implanted, a procedure is performed to assure thatthe leads and/or electrodes are properly implanted in effectivelocations in the body. This programming usually takes place in anoperating room. A navigation session is a procedure to select one ormore effective stimulation sets for a particular patient. Such a sessiongenerally occurs after the leads and/or electrodes are implanted into apatient. Other programming sessions may include a “fitting” procedure,an extensive fitting procedure, a mapping procedure, a follow-upprocedure, and an addition of a program procedure.

During the fitting of an SCS patient either in an operating room orafter implantation, it is often desirable that the level of intensity ofthe stimulation be maintained, even though one or more parameters may bevaried through the fitting session. Stimulus intensity is directlyrelated to the pulse amplitude and pulse width and may be thought of asa perceived energy level delivered to the tissue to be stimulated. Suchparameters that are varied during a fitting procedure include pulsewidth (PW) and amplitude (a). It is important to maintain a relativelyconstant electrical stimulus to the tissue in order to minimize patientdiscomfort during the fitting procedure.

Adjustments to the pulse width may result in changes in the recruitmentof depolarized neural targets. However, changing the pulse width alsoaffects the energy level delivered to the tissue. Therefore, duringfitting it is desirable to change the recruitment but not the energylevel delivered to the tissue. If an energy level or intensity of thestimulus is to be maintained while either amplitude or pulse width ischanged, it is necessary to change both the pulse width and pulseamplitude.

For instance, a clinician adjusting the pulse width must be conscious ofthe effect of the intensity of the stimulus delivered to the tissue.Large changes in stimulation intensity can be uncomfortable for thepatient, as can too high of a stimulation intensity. Likewise, too lowof a stimulation level may not produce sufficient paresthesia toovercome the targeted pain. A clinician manually adjusting the pulsewidth therefore must also adjust pulse amplitude to maintain a constantenergy level until the desired pulse width is achieved. Manuallychanging both the amplitude and the pulse width slows down theprogramming or fitting process. It is necessary to make small changes inpulse width and then make small changes in amplitude until the desiredpulse width is achieved. Since the desired pulse width is often unknownuntil the patient verbally indicates the desired stimulation effect,adjustment is an iterative and sometimes painstaking process.

An automated system of adjusting the pulse width and/or amplitude whilemaintaining relatively constant stimulus intensity is thus desirable.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present inventions, a method ofoperating a tissue stimulation system is provided. The method comprisesplacing at least one electrode in contact with the tissue of a patient.For example, the electrode(s) may be implanted within the patient incontact with the tissue. The method further comprises applying anelectrical stimulus to the electrode(s), e.g., during a fittingprocedure. The electrical stimulus comprises a plurality of pulsesdefined by a pulse width value and an amplitude value. The methodfurther comprises adjusting (e.g., manually) one of the amplitude valueand the pulse width value, and automatically adjusting the other ofamplitude value and the pulse width value in response to the adjustmentof the one of the amplitude value and the pulse width value, such thatan effect of the electrical stimulus (e.g., a perceived stimulationintensity) remains substantially constant. That is, if the amplitudevalue is adjusted, then the pulse width value is automatically adjustedin response thereto, and if the pulse width value is adjusted, then theamplitude value is automatically adjusted in response thereto.Application of the electrical stimulus may be continuously applied tothe tissue as the amplitude and pulse width values are adjusted, ordiscretely applied to the tissue, e.g., between adjustments in theamplitude and pulse width values.

In one method, the other of the amplitude value and the pulse widthvalue is increased if the one of the amplitude value and the pulse widthvalue is decreased, and the other of the amplitude value and the pulsewidth value is decreased if the one of the amplitude value and the pulsewidth value is increased. In another method, the other of the amplitudevalue and the pulse width value is automatically adjusted according to arelationship to maintain the electrical stimulus at a substantiallyconstant intensity level. For example, the relationshipa/(1+k/PW)=a_(x)/(1+k/PW_(x)), wherein k is a constant of pulsefrequency, wherein a_(x) is the adjusted amplitude value, a is theamplitude value, PW_(x) is the adjusted pulse width value, and PW is thepulse width value. In another method, the electrical stimulus is appliedby a pulse generating device, and the adjustment of the other of theamplitude value and the pulse width value is automatically calculated bya programmer and communicated to the pulse generating device.

In accordance with a second aspect of the present inventions, a tissuestimulation system is provided. The tissue stimulation system comprisesa pulse generating device for generating an electrical stimulus (definedin the same manner described above) to provide an effect in a patient(e.g., a perceived stimulation intensity), and at least one electrodefor delivering the electrical stimulus generated by the pulse generatingdevice. The pulse generating device may, e.g., be implantable. Thetissue stimulation system further comprises an interface device capableof allowing a user to adjust (e.g., manually) one of the amplitude valueand the pulse width value, and a programmer capable of programming thepulse generating device to control the generation of the electricalstimulus. The programmer is further capable of calculating an adjustmentin the other of the amplitude value and the pulse width value inresponse to adjusting the one of the amplitude value and pulse widthvalue, such that the effect of the electrical stimulus remainssubstantially constant. This can be accomplished in the same mannerdescribed above.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects of the present invention will be moreapparent from the following more particular description thereof,presented in conjunction with the following drawings wherein:

FIG. 1 depicts a Spinal Cord Stimulation (SCS) system, as an example ofa tissue stimulation system.

FIG. 2 depicts the SCS system of FIG. 1 implanted in a spinal column.

FIG. 3 depicts an exemplary user interface display.

DETAILED DESCRIPTION OF THE EMBODIMENTS

It is to be understood that this invention is not limited to theparticular devices, compositions, methodologies or protocols described,as these may vary. It is also to be understood that the terminology usedin the description is for the purpose of describing the particularversions or embodiments only, and is not intended to limit the scope ofthe present invention which will be limited only by the appended claims.

It must also be noted that as used herein and in the appended claims,the singular forms “a”, “an”, and “the” include plural reference unlessthe context clearly dictates otherwise. Thus, for example, reference toan “electrode” is a reference to one or more electrodes and equivalentsthereof known to those skilled in the art, and so forth. Unless definedotherwise, all technical and scientific terms used herein have the samemeanings as commonly understood by one of ordinary skill in the art.Although any methods, devices, and materials similar or equivalent tothose described herein can be used in the practice or testing ofembodiments of the present invention, the preferred methods, devices,and materials are now described. All publications mentioned herein areincorporated by reference. Nothing herein is to be construed as anadmission that the invention is not entitled to antedate such disclosureby virtue of prior invention.

Although the methods of adjusting a stimulation parameter in response toan adjustment of another stimulation parameter will be described inreference to programming scenarios, the methods are equally applicationto stimulation sessions. A Spinal Cord Stimulation (SCS) system will beused herein as an example of a tissue stimulation system.

The various components of an exemplary SCS system may include animplantable pulse generator (IPG) and programmer used with such system.Implantable components may include an implantable pulse generator, oneor more electrode arrays, and (as needed) one or more extensions toconnect the array(s) to the IPG. Such implantable components, externaldevices and circuitry are more fully described in U.S. Pat. No.6,622,048. Alternatively, a system comprised of an implanted RF receiverand external transmitter, as a pulse generating device in place of anIPG, may be used.

An exemplary Spinal Cord Stimulation (SCS) system 10 is shown in FIG. 1.SCS system 10 comprises an Implantable Pulse Generator (IPG) 12, anoptional lead extension 14, an electrode lead 16, and an electrode array18. The IPG 12 generates stimulation current for implanted electrodesthat make up the electrode array 18. When needed, a proximal end of thelead extension 14 is removably connected to the IPG 12 and a distal endof the lead extension 14 is removably connected to a proximal end of theelectrode lead 16. Alternatively, a proximal end of lead 16 is attacheddirectly to the IPG 12. Electrode array 18 is formed on a distal end ofthe electrode lead 16. The in-series combination of the lead extension14 and electrode lead 16, carry the stimulation current from the IPG 12to the electrode array 18.

The SCS system 10 described in FIG. 1 above is depicted implanted in theepidural space 20 in FIG. 2. The electrode array 18 is implanted at thesite of nerve fibers that are the target of stimulation, e.g., along thespinal cord. Due to the lack of space near the location where theelectrode lead 16 exits the spinal column, the IPG 12 is generallyimplanted in the abdomen or above the buttocks. When needed, the leadextension 14 facilitates locating the IPG 12 away from the electrodelead exit point. Another example of a SCS system that may be used withthe present invention is described in U.S. Pat. No. 6,516,227,incorporated herein by reference in its entirety. Another stimulationsystem is described in U.S. Pat. No. 6,393,325 and related applicationsand issued patents. It is to be emphasized, however, that the inventionherein described may be used with many different types of stimulationsystems, and is not limited to use with the representative SCS system.

Electrical stimulation follows a well-known strength-duration relation.It is known that constant current rectangular pulses, such as biphasicpulses, may stimulate nerve and muscle, wherein the total stimuluscharge Q at threshold increases linearly with the pulse duration d:

Q=k+bd,

wherein k and b are constants. The physicist Lapicque used capacitordischarge pulses to stimulate various tissues, and found that thethreshold peak current, or intensity of the stimulation, I, variedinversely with the pulse duration of discharge, later to be known aspulse width (PW).

The relationship between intensity and pulse width was defined byLapicque:

I=b*(1+k/PW),

wherein I is intensity of an electrical stimulus, b represents therheobase and k/PW stands for the ratio of chronaxie k to pulse width PW.Rheobase is an expression of the minimal strength (amplitude) of anelectrical stimulus that is able to cause excitation of a tissue.Rheobase is generally expressed as a current value. Chronaxie is anexpression of the minimum interval of time (pulse width) necessary toelectrically stimulate a muscle or nerve fiber, using twice the minimumcurrent needed to elicit a threshold response. Chronaxie, k, for thepresent purposes may be a constant of about 200 μs.

The intensity of neurostimulation is directly related to pulse amplitude(a). Using the relationship developed by Lapicque, known as the LapicqueRelationship, pulse amplitude (a) may be substituted for intensity.Therefore, the equation becomes:

a=b*(1+k/PW).

As an example, suppose that 5 times the rheobase is an approximate levelof intensity that is desirable during a stimulation session. Therefore,I=a=5b=b*(1+k/PW). To keep the intensity constant, a/(1+k/PW)=5.Therefore, for each amplitude value (a) and a constant intensity (I), apulse width value may be determined.

If two different values of amplitude (a_(x)) and pulse width (PW_(x))are plugged into the intensity-constant equation:

a/(1+k/PW)=a _(x)/(1+k/PW_(x))=a constant intensity.

To determine the “new” value of amplitude (a_(x)), rearrange theequation to read:

a _(x) =a*(1+k/PW_(x))/(1+k/PW).

To determine the “new” value of pulse width (PW_(x)), rearrange theequation to read:

PW_(x) =k/[(a _(x) /a)*(1+k/PW)−1].

To be consistent in notation, “PW” and “a” without subscripts willdenote “original” or “previous” or “old” values of pulse width andamplitude, respectively. “PW_(x)” and “a_(x)” with subscript “x” willdenote “next” or “new” values of pulse width and amplitude,respectively.

The populations of nerve fibers that are activated by the stimulus arediverse with respect to both effective chronaxie and rheobase. Thisdiversity is in fact exploited by the present adjustment methods toachieve a more efficacious distribution of activated nerve fibers whenpulse width and amplitude are increased and decreased relative to eachother.

Therefore, if one changes the pulse width from PW to PW_(x) and wants toadjust the amplitude (a to a_(x)) without changing the intensity, thenew value of amplitude (a_(x)) would be:

a _(x) =a*(1+k/PW_(x))/(1+k/PW)  (Equation No. 1).

Likewise, if one changes the amplitude (a to a_(x)) and wants to adjustthe pulse width (PW to PW_(x)) without changing the intensity, the newvalue of pulse width (PW_(x)) would be:

PW_(x) =k/[(a _(x) /a)*(1+k/PW)−1]  (Equation No. 2).

Thus, after implanting at least one electrode in a patient fordelivering electrical stimulation pulses generated by a pulse generatingdevice, a pulse generator may apply an electrical stimulation pulse tothe implanted electrode. The stimulation pulse may be defined by atleast a pulse width value and an amplitude value. If a user through aprogrammer adjusts the value of the pulse width, then an automaticadjustment may be made to the amplitude without changing the level ofstimulation intensity or the energy level delivered to the stimulatedtissue. The energy output of the pulse generating device therefore wouldremain substantially constant, while a user adjusts the value of pulsewidth or amplitude and while the programmer automatically adjusts theother parameter.

In order to carry out the adjustment in values to the stimulation pulse,a stimulation programmer may interface with a user device and also withthe implanted pulse generator. Programmers may be in the form of aconventional PC, a laptop, a tablet, a PDA, a monitor, a hand-helddevice, and any other suitable computing means. Alternatively, otherlogical means may used to make the adjustments to the amplitude and/orpulse width, such as a microprocessor within the pulse generator.

A user may make the adjustments to the amplitude or the pulse widththrough a suitable interface such as through a parameter screen. Thescreen may be displayed through any suitable interface device. Interfacedevices may include, but are not limited to, display screens, handhelddevices, monitors, laptops, tablets and PDAs. The interface devices maybe interactive, such as a touch screen. The user may use a mouse,joystick, or stylus in connection with the interface for the inputtingher selections during programming. Thus, the selected user may input oneor more parameter adjustments through the display screens. Theprogrammer then recalculates the other stimulation parameters in orderto maintain substantially constant stimulation intensity. The programmercommunicates the adjustments to the pulse generating device. Theelectrical stimulation may be dynamically applied to the patient'stissue while the amplitude and pulse width values are adjusted, or canbe applied to the patient's tissue between adjustments in the amplitudeand pulse width values. Thus, the electrical stimulus may becharacterized as being continuously applied to the patient's tissue or adiscretely applied to the patient's tissue.

The users may be selected from the group consisting of patient,technician, clinician (such as a nurse, physician, physician'sassistant, etc.), and combinations thereof. In one embodiment, thepatient may be the selected user in order to allow maximum patientcontrol. In another embodiment, the patient and attending clinician mayshare control of the programming session. In another embodiment, such asin an operating room, a clinician controls the programming alone.

The methods of the present invention may be incorporated into any tissuestimulation system, such as any SCS, neural or muscle stimulationsystem. Thus, in another embodiment, a tissue stimulation system isprovided. A system may comprise: (1) a pulse generating device forgenerating electrical stimulation pulses; (2) at least one implantedelectrode for delivering the electrical stimulation pulses generated bythe pulse generating device; (3) a programmer, wherein the programmer iscapable of instructing the pulse generating device to generateelectrical stimulation pulses; and (4) an interface device forcommunicating with the programmer. The electrical stimulation pulses aredefined by several parameters including a pulse amplitude and a pulsewidth. A user may communicate to the stimulation programmer through theinterface device her adjustments of either the pulse width or the pulseamplitude. The programmer may thus calculate the appropriate change tobe made to the pulse width or pulse amplitude, respectively, using theEquation Nos. 1 or 2.

In order to adjust the one or more stimulation parameters, a user mayuse a handheld device or other suitable interface that allowscommunication with the programmer. Any suitable user interface may beincorporated into embodiments of the invention. For example, theinterfaces described in U.S. Pat. No. 6,393,325 may be used or alteredfor the programming sessions described herein.

An example of a programming screen may be the one illustrated in FIG. 3.As seen in FIG. 3, the interface may include three panels, or anycombination or portion of the three panels (301, 302, 303). The user maybe prompted to enter the parameters displayed in the 301 panel. Theseparameters may be set such as pulse width 304 and amplitude or strength306. The user may also adjust the pace 317 of the programming session.Optionally, the parameters may include a rate 305. The interface mayalso have a start 307 and stop 308 button that halts or resumes theprogramming, respectively. In panel 301, a “begin” and an “end” valuefor a parameter are entered. In one embodiment, only a single parameter(e.g., pulse width 304) is highlighted to be auto-adjusted. Hitting thestart 307 button begins the automatic adjustment of the highlightedstimulation parameter from its “begin” value through the minimumincrement to its “end” value. This automated stepped adjustment can alsotake advantage of the auto adjust functionality between the PW and thepulse amplitude. With the interface in panel 302, the user may be ableto manually adjust the pulse width 309, amplitude 310 or pulse rate orfrequency 311, as well as select the between multipolar and monopolarstimulation 318, within the interface displayed at panel 302. In panel303, a user may be able to adjust the amplitude 312. The user mayentirely halt delivery of stimulation pulses, i.e., turn simulation off313. The user may be able to select from 314, 315, and 316, whichcorrespond to sets of electrode combinations to be tested.

One feature of the programming screen is an auto-adjust 319 enablementfeature. If the auto-adjust feature 319 is selected, and if the user ischanging the amplitude, the pulse width will be automatically adjustedby the programmer according to the Lapicque relationship as definedabove. Likewise, if the user is changing pulse width, amplitude will beautomatically adjusted by logical means according to the Lapicquerelationship. By selecting the auto-adjust feature 319, the energyoutput is selected to be substantially constant though a user may beadjusting the amplitude or the pulse width. In the illustratedembodiment, the auto-adjust feature 319 is incorporated into a userinterface as a check box on the programming screen. As another example,this auto-adjust feature 319 may be incorporated into the programminginterface device as a button or key that is pressed when activated andthen depressed when released, such as an Alt key. Other examples areeasily incorporated into a parameter adjustment screen. Additionally,the user may find that the automatic adjustment results in an increasingor decreasing intensity of stimulation. This may be due to the incorrectassumption of the value of chronaxie of 200 us. Chronaxie has been shownto vary with the distance from the electrode to the stimulated nerves.Additionally, the fiber populations stimulated in SCS and otherapplications may have both a narrow range of chronaxies (as in puredorsal column stimulation) or a wider range of chronaxies (as in rootstimulation). So, the chronaxie value used for the automatic adjustmentmay be adjusted by the user using chronaxie adjustment feature 320 onthe screen.

The calculation of pulse width and amplitude by Equation Nos. 1 and 2for a given chronaxie k may maintain stimulus intensity during pulsewidth or amplitude adjustment. The automatic adjustment of thestimulation parameter achieved by the Equation Nos. 1 and 2 may be usedin combination with a manual user adjustment of the other parameter. Auser may, in some alternatives, select and release the auto-adjustfeature to independently adjust the stimulation parameters manually.Once one of the parameters is selected by the user that is suitable, theuser may activate the auto-adjust feature to lock-in the desirableparameter and work therefrom.

The present systems and methods of auto-adjusting one stimulationparameter according to an adjustment of another parameter areparticularly useful in operating room programming. Patient comfort isparticularly critical in the operating room. As explained above, it isdesirable that the level of intensity of stimulation remainsubstantially constant even though the pulse width and amplitude may bechanging. Programming for operating room programming may include theauto-adjust feature.

EXAMPLE

In Tables 1 and 2, a starting PW is 200 μs and a starting amplitude (a)is 4 mA. In Table 1, the pulse width is manually adjusted up to 1000 μsand then back down to 100 μs. The auto-adjust feature, using theLapicque relationship, calculates the corresponding amplitudeadjustments. In Table 1, the amplitude thus is auto-decreased to 2.4 mAand then increased to 6 mA. The constant, k, was set to 200 μs for theExample.

Table 2 illustrates how the pulse width would be auto-adjusted frommanual changes to the amplitude (a). The amplitude in Table 2 is startedat 4 mA, is increased to 6 mA and then decreased to 2.5 mA. The pulsewidth in Table 2, thus is auto-adjusted to decrease from 200 μs to 100μs and then increase to 800 μs.

To avoid round-off problems, in one embodiment, a programmer may store ahigh precision working value of amplitude and pulse width known as“a_(prog)” and “PW_(prog)”, respectively. As illustrated in Tables 1 and2 below, the value of “a_(prog)” and “PW_(prog)” does not change throughthe iterations unless the amplitude is changed by the LapicqueRelationship equation by more than 0.1 mA or the pulse width is changeby more than 10 μs. Thus, the programmed value of the amplitude and thepulse width is not changed to the calculated value of a_(x) and PW_(x),respectively, unless the adjustment calculated by the auto-adjustfeature results in a change of more than 0.1 mA or 10 μs, respectively.

TABLE 1 Change pulse width (PW_(x)), auto-change amplitude (a_(x)) PW aPW_(x) a_(x) a_(prog) 200 4 220 3.818 3.8 220 3.818 240 3.667 3.7 2403.667 260 3.538 3.5 260 3.538 280 3.428 3.4 . . . . . . . . . . . . . .. 940 2.426 960 2.417 2.4 960 2.417 980 2.408 2.4 980 2.408 1000 2.4 2.41000 2.4 980 2.408 2.4 980 2.408 960 2.417 2.4 960 2.417 940 2.426 2.4940 2.426 920 4.5 2.4 . . . . . . . . . . . . . . . 160 4.5 140 4.8574.9 140 4.857 120 5.333 5.3 120 5.333 100 6 6 100 6 80 7 7

TABLE 2 Change amplitude (a_(x)), auto-change pulse width (PW_(x)) a PWa_(x) PW_(x) PW_(prog) 4 200 4.1 190.5 190 4.1 190.5 4.2 181.8 180 4.2181.8 4.3 173.9 170 4.3 173.9 4.4 166.7 170 . . . . . . . . . . . . . .. 5.7 108.1 5.8 105.3 110 5.8 105.3 5.9 102.6 100 5.9 102.6 6 100 100 6100 5.9 102.6 100 5.9 102.6 5.8 105.3 110 5.8 105.3 5.7 108.1 110 5.7108.1 5.6 111.1 110 . . . . . . . . . . . . . . . 2.8 500 2.7 571.4 5702.7 571.4 2.6 666.7 670 2.6 666.7 2.5 800 800 2.5 800 2.4 1000 1000

It has been demonstrated that maintaining the level of stimulationintensity delivered to the stimulation tissue substantially maintainsthe stimulation intensity perceived by the patient at a constant level,e.g., so that the patient always experiences comfortable paresthesiawhile the parameter values are adjusted (i.e., as one of the amplitudevalue and pulse width value is manually adjusted by the user while theother of the amplitude vale and pulse width value is automaticallyadjusted. While maintaining the level of stimulation intensity constantin accordance with Equation Nos. 1 and 2 has been found to be suitablein maintaining the perceived stimulation intensity of the patient at aconstant, there may be other functions that can be used. For example, acharge applied to the tissue may be maintained at a constant level, orthe energy of the stimulation applied to the tissue may be approximatedusing other equations, e.g., pulse width multiplied by the square of thecurrent.

While the amplitude and pulse width values of the stimulus have beenpreviously described as being adjusted to maintain an actual stimulationintensity applied to the tissue, and thus, perceived level ofstimulation, the amplitude and pulse width values may be adjusted tomaintain other stimulation effects. For example, other stimulationeffects that may be automatically maintained at a substantially constantlevel include, but are not limited to, gross area and/or distribution ofparesthesia, features of evoked potentials (e.g., EP amplitude,conduction velocity, etc.), estimates of tissue blood flow and/orperfusion, changes in muscle tone and/or tremor, features of EEG,features of EMG, and metrics of cardiac function (e.g., heart rate, STsegment level, etc.).

In addition, while it been found that the automated adjustment of theamplitude value in view of a manual adjustment in the pulse width value,or vice versa, provides an efficient and effective means for maintaininga stimulation effect, it is possible to automatically adjust otherparameters values of the electrical stimulus (e.g., the frequency rateof the pulses) in response to the manual adjustment of either or both ofthe amplitude value and pulse width value, or vice versa.

For example, the total energy of the electrical stimulus linearly variesin accordance with the frequency rate of the pulses. In this case, aseither or both of the amplitude value and pulse width value are manuallyincreased or decreased, the frequency rate of the pulses mayautomatically be decreased or increased to maintain the stimulationeffect. Or, as the frequency rate of the pulses is manually increased ordecreased, either or both of the amplitude value and pulse width valuecan be automatically decreased or increased to maintain stimulationeffect.

As another example, increasing the length of the rise-time and fall-timeof a stimulus pulse is somewhat equivalent to decreasing pulse width,and vice versa. That is, if the stimulus effect is a result of the totalcharge transfer (area under the current-time curve) per phase, changingthe shape of the pulse to maintain charge transfer constant in responseto a change in amplitude or pulse width is equivalent to changing onlythe pulse width or amplitude. In this case, as the amplitude valueand/or pulse width value is manually increased or decreased, therise-time and/or fall-time of the stimulus pulses may automatically beincreased or decreased to maintain the stimulation effect. Or, as therise-time and/or fall-time of the stimulus pulses is manually increasedor decreased, the amplitude value and/or pulse width value mayautomatically be increased or decreased to maintain the stimulationeffect.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims. Forexample, the methods discussed above are not limited to spinal cordstimulation systems and may be used with many kinds of stimulationsystems such as, but not limited to, those described above, cochlearimplants, cardiac stimulation systems, peripheral nerve stimulationsystems, muscle tissue stimulation systems, brain stimulation systemsand micro stimulators.

1. (canceled)
 2. A method, comprising: delivering electrical current toat least one electrode that is in contact with tissue of a patient toprovide a stimulation effect having a stimulation intensity, whereinadjustable stimulation parameters are used to define the deliveredelectrical current that provides the stimulation effect having thestimulation intensity; and adjusting at least a first one of thestimulation parameters, and then automatically adjusting at least asecond of the stimulation parameters in response to adjusting the atleast the first of the stimulation parameters, wherein the at least thesecond of the stimulation parameters is automatically adjusted tomaintain the stimulation intensity of the stimulation effect using adefined relationship of the adjustable stimulation parameters to thestimulation intensity.
 3. The method of claim 2, wherein adjusting theat least the first one of the stimulation parameters includes adjustingan amplitude.
 4. The method of claim 3, wherein: delivering electricalcurrent includes delivering pulsed electrical current that has pulsesthat have a pulse width; the defined relationship of the adjustablestimulation parameters to the stimulation intensity includes a definedrelationship of the amplitude and the pulse width to the stimulationintensity; and automatically adjusting the at least the second of thestimulation parameters includes automatically adjusting the pulse widthusing the defined relationship.
 5. The method of claim 2, whereindelivering electrical current includes delivering pulsed electricalcurrent that has pulses that have a pulse width, and adjusting the atleast the first one of the stimulation parameters includes adjusting thepulse width.
 6. The method of claim 5, wherein: the defined relationshipof the adjustable stimulation parameters to the stimulation intensityincludes a defined relationship of an amplitude and the pulse width tothe stimulation intensity; and automatically adjusting the at least thesecond of the stimulation parameters includes automatically adjustingthe amplitude of the pulses using the defined relationship.
 7. Themethod of claim 2, wherein: delivering electrical current includesdelivering pulsed electrical current that has pulses that have a pulsewidth and that has a pulse frequency and an amplitude; the definedrelationship of the adjustable stimulation parameters to the stimulationintensity includes a defined relationship of the amplitude, the pulsewidth and the pulse frequency to the stimulation intensity; adjustingthe at least the first one of the stimulation parameters includesadjusting the pulse frequency of the pulsed electrical current; andautomatically adjusting the at least the second of the stimulationparameters includes using the defined relationship to automaticallyadjust the amplitude, or automatically adjust the pulse width, orautomatically adjust both the amplitude and the pulse width.
 8. Themethod of claim 2, wherein delivering electrical current includesdelivering pulsed electrical current that has pulses that have a pulsewidth and that has a pulse frequency and an amplitude; the definedrelationship of the adjustable stimulation parameters to the stimulationintensity includes a defined relationship of the amplitude, the pulsewidth and the pulse frequency to the stimulation intensity; adjustingthe at least the first one of the stimulation parameters includesadjusting the amplitude; and automatically adjusting the at least thesecond of the stimulation parameters includes using the definedrelationship to automatically adjust the pulse frequency.
 9. The methodof claim 2, wherein the delivered electrical current includes deliveringpulsed electrical current that has pulses that have a pulse width andthat has a pulse frequency and an amplitude; the defined relationship ofthe adjustable stimulation parameters to the stimulation intensityincludes a defined relationship of the amplitude, the pulse width andthe pulse frequency to the stimulation intensity; adjusting the at leastthe first one of the stimulation parameters includes adjusting the pulsewidth; and automatically adjusting the at least the second of thestimulation parameters includes using the defined relationship toautomatically adjust the pulse frequency.
 10. The method of claim 2,wherein the at least the first of the stimulation parameters or the atleast the second of the stimulation parameters includes a pulse risetime, or a pulse fall time, or both the pulse rise time and the pulsefall time.
 11. A non-transitory machine-readable medium includinginstructions, which when executed by a machine, cause the machine to:instruct a stimulator to deliver electrical current to at least oneelectrode that is in contact with tissue of a patient to provide astimulation effect having a stimulation intensity, wherein adjustablestimulation parameters are used to define the electrical current thatprovides the stimulation effect having the stimulation intensity; andautomatically respond to a manual adjustment of at least a first one ofthe stimulation parameters by automatically adjusting at least a secondof the stimulation parameters to maintain the stimulation intensity ofthe stimulation effect using a defined relationship of the adjustablestimulation parameters to the stimulation intensity.
 12. Thenon-transitory machine-readable medium of claim 11, wherein theautomatically respond includes automatically respond to an adjustment ofan amplitude by automatically adjusting a pulse width to maintain thestimulation intensity of the stimulation effect.
 13. The non-transitorymachine-readable medium of claim 11, wherein the automatically respondincludes automatically respond to an adjustment of a pulse width byautomatically adjusting an amplitude to maintain the stimulationintensity of the stimulation effect.
 14. The non-transitorymachine-readable medium of claim 11, wherein: the deliver electricalcurrent includes deliver pulsed electrical current that has pulses andthat has a pulse frequency and an amplitude; the defined relationship ofthe adjustable stimulation parameters to the stimulation intensityincludes a defined relationship of the amplitude, the pulse width andthe pulse frequency to the stimulation intensity; and the automaticallyrespond includes automatically respond to an adjustment of a pulsefrequency by automatically adjusting the amplitude to maintain thestimulation intensity of the stimulation effect using the definedrelationship.
 15. The non-transitory machine-readable medium of claim11, wherein: the deliver electrical current includes deliver pulsedelectrical current that has pulses that have a pulse width and that hasa pulse frequency; the defined relationship of the adjustablestimulation parameters to the stimulation intensity includes a definedrelationship of the amplitude, the pulse width and the pulse frequencyto the stimulation intensity; and the automatically respond includesautomatically respond to an adjustment of a pulse frequency byautomatically adjusting the pulse width to maintain the stimulationintensity of the stimulation effect using the defined relationship. 16.The non-transitory machine-readable medium of claim 11, wherein: thedeliver electrical current includes deliver pulsed electrical currentthat has pulses that have a pulse width and that has a pulse frequencyand an amplitude; the defined relationship of the adjustable stimulationparameters to the stimulation intensity includes a defined relationshipof the amplitude, the pulse width and the pulse frequency to thestimulation intensity; and the automatically respond includesautomatically respond to an adjustment of a pulse frequency byautomatically adjusting both the amplitude and the pulse width tomaintain the stimulation intensity of the stimulation effect using thedefined relationship.
 17. The non-transitory machine-readable medium ofclaim 11, wherein the at least the first of the stimulation parametersor the at least the second of the stimulation parameters includes apulse rise time, or a pulse fall time, or both the pulse rise time andthe pulse fall time.
 18. A system, comprising: a programmer configuredto instruct a stimulator to deliver electrical current to at least oneelectrode that is in contact with tissue of a patient to provide aneffect, wherein adjustable stimulation parameters are used to define theelectrical current that provides the stimulation effect having thestimulation intensity; wherein the programmer is configured toautomatically respond to a manual adjustment of at least a first one ofthe stimulation parameters by automatically adjusting at least a secondof the stimulation parameters to maintain the stimulation intensity ofthe stimulation effect using a defined relationship of the adjustablestimulation parameters to the stimulation intensity.
 19. The system ofclaim 18, wherein the defined relationship between the adjustablestimulation parameters and the stimulation intensity includes a definedrelationship of an amplitude and a pulse width to the stimulationintensity, and the controller is configured to automatically respond toa manual adjustment of the amplitude by automatically adjusting thepulse width using the defined relationship to maintain the stimulationintensity of the stimulation effect.
 20. The system of claim 18, whereinthe defined relationship of the adjustable stimulation parameters to thestimulation intensity includes a defined relationship of an amplitudeand a pulse width to the stimulation intensity, and the controller isconfigured to automatically respond to a manual adjustment of the pulsewidth by automatically adjusting the amplitude to maintain thestimulation intensity of the stimulation effect.
 21. The system of claim18, wherein: the stimulator is configured to deliver pulsed electricalcurrent that has pulses that have a pulse width and that has a pulsefrequency and an amplitude; the defined relationship of the adjustablestimulation parameters to the stimulation intensity includes a definedrelationship of the amplitude, the pulse width and the pulse frequencyto the stimulation intensity; and the controller is configured toautomatically respond to a manual adjustment of the pulse frequency byautomatically adjusting both the amplitude and the pulse width tomaintain the stimulation intensity of the stimulation effect.